During no load, hot-hold periods of operation of fuel cell stacks, there is a need to control potentials in the stack anodes and cathodes. If the electrical potential in the cells of a stack of, for example, phosphoric acid fuel cells is allowed to exceed certain limits, on the order of about 0.8 volts, the electrodes of the fuel cells can be damaged due to corrosion. Such electrical potential control is needed during off-power or non-operating periods, during power plant shutdown, start-up, and hot-holds.
U.S. Pat. No. 5,013,617 granted May 7, 1991 to G. W. Scheffler discloses a method and system for purging fuel cell stack components, and for passivating the anodes and cathodes of the cells in a fuel cell stack during hot, no load periods. The aforesaid patent suggests the use of nitrogen gas to purge the reformer and anodes, and the use of a mixture of oxygen and nitrogen gas to purge the cathodes. After the initial purging operation, the anodes are passivated by a stream of nitrogen gas, and the cathodes are passivated by a stream of a mixture of oxygen and nitrogen. U.S. Pat. No. 5,013,617 is hereby incorporated herein in its entirety. Fuel cell anodes can also be passivated by a stream of hydrogen gas. This latter solution to the passivation problem is not desirable due to the need to store the passivation hydrogen gas at the power plant sites.